This invention relates to thermoplastic elastomer compositions comprising blends of graft copolymers, polyvinyl chloride (PVC) and plasticizers. The compositions require no curing or vulcanization to develop elastomeric properties. The compositions thus remain thermoplastic and can be repeatedly remolded or extruded.
The term "thermoplastic elastomer" has generally been applied to elastomers that can be readily processed and reprocessed by conventional belt processing equipment by virtue of the fact that such elastomers are not cured or vulcanized. The reprocessability of these elastomers compared with conventional cured or thermoset rubbers results in a great reduction in loss due to scrap, with consequent economic benefits for the processor. A variety of such materials have been introduced in recent years such as thermoplastic polyesters, styrene block copolymers, and thermoplastic olefin-rubber blends. Typical of such materials are the styrene-butadiene-styrene block copolymers sold as Kraton brand elastomers by the Shell Chemical Co. and the Hytrel brand polyester elastomers sold by DuPont. Many of these elastomers have found wide application in consumer goods such as in shoe soling formulations and the like, as well as in such industrial applications as wire coating, hose and tubing, electrical connectors and automotive parts.
Currently available thermoplastic elastomers suffer some disadvantages in use. In particular, formulations based on olefinic resins including SBS block copolymers exhibit poor resistance to hydrocarbon solvents and low abrasion resistance which may limit their use in particular environments. Additionally, adhesion to dissimilar materials is poor, and a surface chlorination or other primer treatment is often needed to increase adhesive bonding between, for example, a molded shoe sole formed of such materials and a synthetic shoe upper. Primer treatment of the surfaces of molded goods is also needed where the part is to be painted, which further increases the production cost of such goods.
High rubber content graft copolymers, by which is meant graft polymers prepared by graft-polymerizing rigid monomers in the presence of rubbery polymeric substrates and comprising greater than 50 wt% rubbery substrate polymer, are widely used as impact modifiers for thermoplastics such as styrene resins, styrene-acrylonitrile (SAN) resins, PVC resins and the like. Typical of such materials are graft copolymers of styrene and acrylonitrile and optionally methylmethacrylate on diene rubber substrates such as the Blendex brand modifiers sold by Borg-Warner Chemicals, Inc. and graft copolymers of methacrylates on acrylic rubber substrates and of methacrylates on diene rubber substrates sold as modifier resins by Rohm and Haas. A wide variety of such graft copolymers is readily available commercially. In general, rubbery graft copolymer impact modifiers are employed at levels of less than 30 wt% and often at less than 10 wt% in blends with rigid resins to provide high impact thermoplastics. Although often referred to as rubbery modifiers, these copolymers for the most part are not truly elastomeric and do not exhibit useful elastomeric properties without further modification and vulcanization. These materials therefore are not considered to be thermoplastic elastomers.
The modification of nitrile rubbery stocks by adding PVC together with a conventional PVC plasticizer has long been practiced in the rubber compounding art. A minor proportion of PVC, usually less than 33 wt%, is used to impart increased sunlight and ozone resistance to nitrile rubber, together with improved abrasion and tear properties. Such formulations find use in wire and cable coverings and in the production of hose and tubing, as well as in shoe sole formulations. These blends are, for most applications, normally vulcanized to provide elastomeric character and therefore are not considered to be thermoplastic elastomers.
Thermoplastic elastomer formulations based on high rubber graft copolymers would be a useful advance in the art. The graft copolymers are widely available and readily produced by a variety of well known and economical processes. As is well known, graft copolymers may be readily modified by varying the type and proportion of monomers used in their preparation to selectively improve such characteristics as abrasion and solvent resistance, adhesion, weatherability and the like. Elastomers based on graft copolymers could thus find application in the production of molded and extruded goods to meet a wide variety of environmental requirements including shoe soling, extruded hose and tubing, wire and cable insulations, the production of flexible cord, automotive parts and the like.